Reverse Every K Nodes - c

Question

Every k nodes form a segment. If the last few nodes are less than K, then you can ignore them.
Write a reverseKnodes() which reserves every segment in the linked list.
The function prototype is given as follow: void reversekNodes(ListNode** head, int k);
Input format:
The 1st line is the k
The 2nd line is the data to create the linked list and ends with a non-digit symbol Example:
Input: 3 1 2 3 4 5 6 7 8 9 10 a
Output: 3 2 1 6 5 4 9 8 7 10
#include <stdio.h>
#include <stdlib.h>
struct _listNode
{
int item;
struct _listNode *next;
};
typedef struct _listNode ListNode;
void printList (ListNode * head);
void deleteList (ListNode ** ptrHead);
void reverseKNodes (ListNode ** head, int K);
int
main ()
{
ListNode *head = NULL, *temp;
int i = 0;
int K = 0;
scanf ("%d", &K);
while (scanf ("%d", &i))
{
if (head == NULL)
{
head = (ListNode *) malloc (sizeof (ListNode));
temp = head;
}
else
{
temp->next = (ListNode *) malloc (sizeof (ListNode));
temp = temp->next;
}
temp->item = i;
}
temp->next = NULL;
reverseKNodes (&head, K);
printList (head);
deleteList (&head);
return 0;
}
void
printList (ListNode * head)
{
while (head != NULL)
{
printf ("%d ", head->item);
head = head->next;
}
printf ("\n");
}
void
deleteList (ListNode ** ptrHead)
{
ListNode *cur = *ptrHead;
ListNode *temp;
while (cur != NULL)
{
temp = cur->next;
free (cur);
cur = temp;
}
*ptrHead = NULL;
}
void
reverseKNodes (ListNode ** head, int K)
{
struct _listNode *reverse (struct _listNode *head, int k){
if (!head)
return NULL;
struct _listNode* cur = head;
struct _listNode* next = NULL;
struct _listNode* prev = NULL;
int count = 0;
while (cur != NULL && count < K)
{
next = cur->next;
cur->next = prev;
prev = cur;
cur = next;
count++;
}
if (next != NULL)
head->next = reverse(next, K);
return prev;
}
}
I am not allowed to change anything else other than the void function for reverseKNodes and I know I have done it wrong but I dont know where I went wrong. Could someone help me please?

Answer 1

A few issues with nested functions ...
Nested functions aren't standard C.
Are of limited benefit.
They add extra overhead to invoke.
So [please] don't use them.
And, your usage of one is incorrect. In reverseKNodes, you define reverse but never call it. So, reverseKNodes is a no-op.
So, to fix, move reverse out of the body of reverseKNodes and place it above. Then, actually invoke it in reverseKNodes:
void
reverseKNodes(ListNode **head, int K)
{
*head = reverse(*head, K);
}
Here is the full refactored code. For aid in debug (e.g. with gdb), I added code to allow the program to take an input file as an argument.
#include <stdio.h>
#include <stdlib.h>
struct _listNode {
int item;
struct _listNode *next;
};
typedef struct _listNode ListNode;
void printList(ListNode *head);
void deleteList(ListNode **ptrHead);
void reverseKNodes(ListNode **head, int K);
int
main(int argc,char **argv)
{
ListNode *head = NULL, *temp;
int i = 0;
int K = 0;
--argc;
++argv;
FILE *xf;
do {
if (argc != 1) {
xf = stdin;
break;
}
xf = fopen(*argv,"r");
if (xf != NULL)
break;
perror(*argv);
exit(1);
} while (0);
fscanf(xf,"%d", &K);
while (fscanf(xf,"%d", &i)) {
if (head == NULL) {
head = (ListNode *) malloc(sizeof(ListNode));
temp = head;
}
else {
temp->next = (ListNode *) malloc(sizeof(ListNode));
temp = temp->next;
}
temp->item = i;
}
temp->next = NULL;
reverseKNodes(&head, K);
printList(head);
deleteList(&head);
return 0;
}
void
printList(ListNode *head)
{
while (head != NULL) {
printf("%d ", head->item);
head = head->next;
}
printf("\n");
}
void
deleteList(ListNode **ptrHead)
{
ListNode *cur = *ptrHead;
ListNode *temp;
while (cur != NULL) {
temp = cur->next;
free(cur);
cur = temp;
}
*ptrHead = NULL;
}
struct _listNode *
reverse(struct _listNode *head, int K)
{
if (! head)
return NULL;
struct _listNode *cur = head;
struct _listNode *next = NULL;
struct _listNode *prev = NULL;
int count = 0;
while (cur != NULL && count < K) {
next = cur->next;
cur->next = prev;
prev = cur;
cur = next;
count++;
}
if (next != NULL)
head->next = reverse(next, K);
return prev;
}
void
reverseKNodes(ListNode **head, int K)
{
*head = reverse(*head, K);
}
Here is the program output:
3 2 1 6 5 4 9 8 7 10